1. Field of the Invention
The present invention generally relates to a photocell having a photosensitive element incorporated into a semiconductor substrate and an entry face through which the light enters and more particularly related to the means of optical coupling between the entry face and the photosensitive element.
2. Description of Related Art
Many optical components incorporate a photocell. Such components are, for example, optical detectors consisting of a single element, or photocell, sensitive to the light incident on its entry face. They may also be optical detectors formed from a matrix photocells juxtaposed alongside one another, the respective entry faces of which lie in one and the same plane. In this case, the photocells may be distinguished by their position within the matrix, for example for detecting the position of a light spot on the surface of this matrix. They may also be distinguished by the wavelength of the light to which each of them is sensitive, or distinguished by a combination of the sensitivity wavelength and of the position of the photocell within the matrix, for example for detecting polychromatic light images.
In such components, the photocell has small dimensions in order to achieve good spatial resolution. For example, it has a light entry face having the approximate dimensions of 5 μm×5 μm (μm meaning micrometer). Consequently, the amount of light entering the photocell is limited by the size of this entry face.
The photocell has in general an overall cylindrical shape, the cross section of which in a plane orthogonal to the main axis of this cylinder can vary in shape. When the photocell is shown in cross section in a plane containing its main axis, this axis is in general shown as lying along the vertical, with the entry face at the top and the sensitive surface at the bottom. Hereafter, the terms “lower” and “upper” used in relation to the designation of the constituent elements of the photocell, or of the photosensitive matrix, are so by reference to this convention. The photocell is thus bounded in its upper part by the entry face and in its lower part by a semiconductor substrate whose inner surface with respect to the photocell is substantially plane and parallel to the entry face. This semiconductor substrate carries, on part of its surface, the photosensitive element. At this element, in order to increase the sensitivity, the inner surface of the substrate may be made particularly rough, but it maintains a substantially plane overall geometry. An optical device may also be placed on or near the plane surface of the photosensitive element. This optical device may consist of anti-reflection layers or of a microlens matched to the surface of the photosensitive element.
The photosensitive element is in general a surface portion of the semiconductor substrate, the surface portion being especially treated in order to fulfill this function. However, this surface of the substrate includes, inside the photocell, apart from the above photosensitive element, a surface portion occupied by electronic components integrated into the photocell.
These components form part of the means for reading and controlling the photosensitive photocell. The photosensitive element therefore has a photosensitive surface whose area is less than the area of the entry face. For example, the dimensions of the photosensitive surface are 2 μm×2 μm for an entry face of 5 μm×5 μm.
In practice, as a result, in the case of incident light in the form of a beam directed approximately normal to the entry face, that portion of the light entering the photocell which illuminates the semiconductor substrate at the electronic components is lost, because it does not reach the photosensitive element. This loss reduces the sensitivity of the photocell, particularly for low incident light intensities, compared with theoretical sensitivity defined by the size of the entry face for light to enter the photocell.
To overcome this drawback, a convergent microlens is usually placed at the entry surface in order to focus, onto the photosensitive element, the light entering through the entire surface of the entry face. In this case, the reduction in sensitivity of the photocell is to a large part avoided.
However, this use of a microlens is no longer suitable when the distance between the entry face and the surface of the semiconductor substrate is too great. For the photocell dimensions mentioned above, this distance may be between 6 μm and 10 μm. This is especially the case when the electronic components placed on the semiconductor substrate in the immediate vicinity of the photosensitive element require several levels of metallization above the surface of this substrate, for example three or more metallization levels. In this configuration, the microlens must have a sufficiently large focal length, approximately equal to the distance which separates the microlens from the surface of the photosensitive element, to make the light converge on this surface.
For such small dimensions of the entry face, it is difficult to produce microlenses having sufficiently large radii of curvature. Microlenses having focal lengths which are too short cause the light entering the photocell via the entry face to converge above the surface of the photosensitive element, several micrometers therefrom. The light then diverges again between its point of convergence and the surface of the photosensitive element. The desired effect of using the microlens is therefore reduced, if not lost in its entirety. Accordingly, a need exists to overcome the problem with the light divergence in photocells and especially photocells with microlenses.